Hard disk drive incorporating rotating magnetic disks is commonly used for storing data in the magnetic media formed on the disk surfaces, and a movable slider including read sensors are generally used to read data from tracks on the disk surfaces.
Presently, magnetoresistive (MR) read sensor, commonly referred to as MR sensor, is the prevailing read sensor because of its better capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. Now, several types of MR sensors have been widely used by disk drive manufacturers in succession. One is anisotropic magnetoresistive (AMR) sensor, which makes the angle between the magnetization direction and the direction of sense current flowing through the MR element change and, in turn, cause a change the resistance of the MR element and a corresponding change in the sensed current or voltage. Another type is giant magnetoresistive (GMR) sensor manifesting the GMR effect. The GMR effect is a phenomenon that the magnetoresistive ratio (MRR) will change under an external time-changing magnetic field. The GMR sensor includes two ferromagnetic layers and a non-ferromagnetic layer sandwiched between the two ferromagnetic layers. The resistance of the non-ferromagnetic layers varies with the magnetic moments of the ferromagnetic layers, the conduction electrons and the spin-dependent scattering. Still another type of MR sensor is tunnel magnetoresistive (TMR) sensor, which includes a magnetic tunnel junction (MTJ) where the tunneling magneto-resistance effect (TMR effect) occurs. The TMR sensor has become the mainstream MR sensor due to its more remarkable change of MRR by replacing AMR sensor and GMR sensor.
As shown in FIG. 1, the TMR sensor 100 includes two shields 120, two hard magnets 130 and a magnetic tunnel junction (MTJ) 110 which is sandwiched between the shields 120 and the hard magnets 130. Referring to FIG. 2, the MTJ structure 110 includes a first ferromagnetic layer 111, a second ferromagnetic layer 112 and an anti-ferromagnetic (AFM) layer 113 which is formed in physical contact with the second ferromagnetic layer 112 to provide exchange bias magnetic field by exchange coupling at the interface of the layers. The magnetization direction in the second ferromagnetic layer 112 is constrained or maintained by the exchange coupling, thus the second ferromagnetic layer 112 is also called “pinned layer” 112. In general, the magnetization direction of the first ferromagnetic layer 111 is controlled by longitudinal bias magnetic field which is produced by the hard magnets 130. When an external magnetic field applied onto the TMR sensor is strong enough to compensate the longitudinal bias magnetic field, the magnetization direction of the first ferromagnetic layer 111 is free to rotate in response to the external applied magnetic field, thus the first ferromagnetic layer 111 is also called “free layer”. The direction of the magnetization in the free layer 111 changes between parallel and anti-parallel against the direction of the magnetization in the pinned layer 112, and hence the tunneling magneto-resistance effect (TMR) characteristics are obtained.
In a TMR sensor, to suppress Barkhousen noise due to non-continuous magnetization by fluctuations or displacements of the magnetic domain boundaries, the bias magnetic field (longitudinal bias magnetic field) for controlling the magnetic domains is applied toward the longitudinal direction (track width direction). If this longitudinal bias magnetic field applied to the free layer is small, Barkhousen noise will be easily occurred. If the longitudinal bias magnetic field is large, the change in the magnetization direction of the free layer becomes difficult causing the sensing sensitivity of TMR sensor to be degraded. Therefore, it is necessary to apply an optimum amount of the longitudinal bias magnetic field in a TMR sensor. In order to implement it, it is very important to know the strength of the longitudinal bias magnetic field of the TMR sensor.
At present, a popular method for measuring longitudinal bias magnetic field in a TMR sensor is provided. It includes a step of applying high bias current or external heat source onto the TMR sensor so as to break the exchange coupling between the pinned layer and the AFM layer. Since the exchange coupling in the TMR sensor is so strong that the applied bias current or external heat source needs to be higher. However, as well known, higher bias current or heat source will bring higher temperature rise, under high temperature, the strength of longitudinal bias magnetic field will become lower and in result the longitudinal bias magnetic field can not be measured accurately. In addition, high temperature will induce high temperature noise, thereby destructing the performance of the TMR sensor.
Hence, it is desired to provide an improved method for accurately measuring longitudinal bias magnetic field in TMR sensor of a magnetic head.